Conventionally, most of switching power supply devices has an overcurrent protection circuit for forcibly stopping a switching operation of an output transistor when an overcurrent is detected. A typically used type of the overcurrent protection circuit is a pulse-by-pulse type in which a forcible-stop and self-return of a switching operation is repeated at every period.
However, sometimes an output fault (grounded output or the like) that an extremely large amount of overcurrent flows occurs, in the pulse-by-pulse type overcurrent protection circuit in which a forcible-stop and self-return (resume) of the switching operation is repeated. In this case, even though an ON time is shortened to a minimum at every switching period, limiting an overcurrent is still delayed. Thus, an inductor current is continually increased, leading to breakdown of a switching output circuit.